1. Field of the Invention
The present invention relates to an image formation apparatus and a control method thereof which perform image formation by developing a latent image formed on an image support body with use of a development unit and then transferring the developed image onto a fed recording medium.
2. Related Background Art
Conventionally, an electrophotographic system has been known in a color image formation apparatus. In an image formation process of the electrophotographic system, initially, a photosensitive drum is uniformly charged by a charging unit, and an electrostatic latent image is formed by a laser or an LED (light emitting diode). Then the formed electrostatic latent image is developed by using toner to form a toner image, and the formed toner image is transferred onto a recording member such as a recording sheet of paper (referred as recording sheet hereinafter). Such an operation is performed for each of yellow (Y), magenta (M), cyan (C) and black (K), and the toner images for these colors disposed on the sheet are fixed thereto by heat, whereby a color image is formed. In this process, after the toner image is transferred onto the sheet, the residual toner on the photosensitive drum is eliminated by a cleaning unit.
It has been demanded in recent years to reduce a manufacturing cost of the color image formation apparatus and also to downsize the apparatus itself. For this reason, it has been proposed a so-called cleanerless apparatus in which any cleaning unit is not provided in the vicinity of the image support body such as the photosensitive drum.
In such the cleanerless apparatus, there are provided several methods to eliminate the residual toner on the photosensitive drum. In one method, for example, a contact-type charging unit disposed in the vicinity of the photosensitive drum once captures small-quantity residual toner on the drum after the transferring, changes an electrostatic characteristic of the captured toner, brings back the characteristic-changed toner to the drum, and then the development unit collects the brought-back toner and reuses it. According to such the method, it is controlled to collect the residual toner on the photosensitive drum during a printing job or during postrotation at the end of the printing job.
During sheet-to-sheet interval in the printing job or during the drum postrotation at the end of the printing job, the residual toner is captured by the charging unit, the captured toner is ejecting, and the ejected toner is then collected by the developing unit. However, in such operations, to eject the toner from the charging unit (i.e., to bring back the toner once captured by the charging unit to the photosensitive body) can not overtake. For this reason, the toner is mixed with a ferrite carrier acting as a dielectric brush in the charging unit, whereby it is impossible to maintain charging performance of the charging unit. As a result, there has been a problem that quality of a finally formed image is deteriorated.
In order to solve this problem, it has been thought to increase the frequency of cleaning operations. However, if the frequency of cleaning operations is increased, there occurs a problem that to unnecessarily perform the cleaning operation using the contact-type charging unit deteriorates the photosensitive drum.
In a case where it is necessary to collect the residual toner on one photosensitive drum during the printing Job, if the residual toners on the other drums are collected always at identical timing, an unnecessary cleaning operation is performed to the photosensitive drums for the colors other than black on condition that continuous printing is performed in a black/white mode and any image formation is not performed on the drums for the colors other than black. Thus, there occurs a problem that the photosensitive drum is deteriorated.